Acoustic helicoidal beams have an azimuthal phase gradient and an axial null in amplitude. The sign and magnitude of the azimuthal phase gradient determine the helicity and topological charge of the beam. Beams with a unit-magnitude topological charge have been generated with a four-quadrant sectored array, adjacent quadrants being driven with a 90 deg phase offset [B. T. Hefner and P. L. Marston, J. Acoust. Soc. Am. 106, 3313–3316 (1999)]. The scattering by symmetric targets placed on the axis of such beams preserves the helicity and the axial null [P. L. Marston, J. Acoust. Soc. Am. 124, 2905–2910 (2008); F. G. Mitri, Ann. Phys. 323, 2840–2850 (2008)]. The present work shows how to rapidly alter (or modulate) the helicity with an appropriately timed reversal of the excitation of a single pair of opposite quadrants. A simple four-element helicity-selective detector (using appropriately offset elements) was also demonstrated. These beams may be useful for digital acoustic communication and they have a modulated axial angular momentum flux. They should also generate a modulated acoustic torque on targets. [Work partially supported by ONR.]

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A numerical simulation for porous and non-porous infrasonic arrays has been developed to aid in the analysis of the response of these systems. For design flexibility, the simulation makes use of differential T-elements, which may be used as building blocks to assemble pipes and junctions of various shapes and sizes. The simulation adds to the capabilities of a previously developed model [Alcoverro and Le Pichon, J. Acoust. Soc. Am. 117, 1717 (2005)], by including variable tube porosity, a calculation of wall-losses based on a thermo-viscous function, a calculation of the change in response as the azimuth and elevation of the incoming wave are varied, and a calculation of the wave shape as it propagates through the array in the time domain. [Research funded by the Applied Research Laboratory Educational and Foundational Fund.]

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